Ion Tank question

Just got all the stuff for my ion this morning, including a Halo B!!! But I was wondering if I took the tank off (the tank still has some air left in it) would all the air leak out? and how long could I keep a compressed air tank on the Ion with the tank still having air on it 2-3hrs? or much longer??

If you don't have a on/off, then you need to buy one. Buy whatcha do is unscrew the tank off untill you hear air start to leak, then shoot a couple of times and it shouldn't blow air in your face. Make sure that there's no balls in the breech when you do it!!! EDIT::: I don't think that it's good to leave a tank on there long. It's a lot of pressure

How long? Could I leave it on for about 4hrs? or is that too long. EDIT: The gauge on the tank is a little below the first line in the 0 rectangle (its what it looks like on the tank guage) so , its not too much pressure I guess, but I just want to save some air to show my dad how it shoots!

Ill probably do that gtospec35, since I want a top quality one, but dont have money... and I doubt used will make it work any worse. Oh also, im asking here since my last thread about it was closed, how do I install the QEV for the ion? I was told the Ultimate sticky, and wtf is that? I looked in the FAQ and its not there, theres an "Installation of QEV" kinda thing, but they dont tell you how to install it. All i need is a link on how to install it. I found a video posted on this forum on how to instlal it, but for some reason the top still wont come off. I took out all the screws the vid showed, but still doesnt come off.